The goal of this proposal is to better understand the role of microglia in regulating the structure and function of the brain under physiological and pathological conditions. Microglia are the resident immune cells of the central nervous system and display highly motile processes occupying a non- overlapping territory. Under physiological conditions, microglia may participate in the development and plasticity of neural circuits and monitor the brain's microenvironment for damage signals. Under pathological conditions, microglia may contain tissue damage, phagocytose cellular debris, and promote neuronal plasticity. Although microglia have been implicated in many physiological and pathological processes, their functions in the central nervous system remains elusive. Hampering efforts to delineate the precise role of microglia is the lack of tools to specifically perturb microglial function in vivo. We have recently generated mice with a targeted gene insertion allowing for the expression of tamoxifen-inducible Cre recombinase in CX3CR1 expressing microglial cells and peripheral myeloid cells. By taking advantage of different turnover rates of microglia and peripheral myeloid cells, we propose to establish an approach that will allow us, for the first time, to specifically perturb microglial functions in the living mice. By specifically ablating microglia or removing brain- derived neurotrophic factor (BDNF) from microglia in the living mice, we will elucidate the functions of microglia and microglial BDNF in synapse development and learning-dependent synaptic plasticity. In addition, we will reveal the role of microglial activation in controlling neuronal damage, glial scar formation and synaptic remodeling after traumatic brain injury. As activated microglia are involved in almost every pathological condition in the brain, the proposed studies of identifying precise functions of microglia will provide important insights for the understanding and treatment of many neurological diseases.